A. Technical Field
This invention relates to high efficiency power plants, and in particular to a light-weight and efficient solar thermal power plant based on a Rankine-Brayton hybrid thermodynamic cycle, for powering a solar-thermal aircraft.
B. Description of the Related Art
Working prototypes have demonstrated the feasibility and utility of solar powered aircraft. Many if not most solar powered aircraft, however, rely on the photovoltaic conversion of sunlight to electricity to power an electric motor-based propulsion system. Batteries (or electrolyzer, gas storage, and regenerative fuel cells) are carried onboard the aircraft to store electrical energy and keep the aircraft aloft during the night, so that flight time is not limited by fuel supply as it is on a conventional aircraft. It is widely recognized, however, that onboard voltaic batteries or electrolyzer, gas storage, and regenerative fuel cell systems impose a substantial weight burden for all aircraft, and especially for high-altitude, long range aircraft.
Another problem associated with photovoltaic power generation arrangements for aircraft, especially high-altitude, long-range aircraft, is the need to orient/point the photovoltaic solar cells to face the sun. Having wing-mounted arrays of solar panels can limit the efficiency of the collection of solar power, especially at dawn and dusk, as sunlight seldom strikes the solar panels “face on”. Thus in order to achieve a direct angle of impingement, the aircraft could be “banked” (i.e. laterally incline the aircraft, such as by elevating one wing or side higher or lower in relation to the opposite wing or side) in order to face the sun. This practice is disclosed by U.S. Pat. No. 4,415,133 to Phillips, as well as U.S. Pat. No. 5,518,205 to Wurst, et al. Conventional aircraft, however, cannot maintain straight flight at a large bank angle for extended lengths of time. Moreover, a related problem is the significant restriction on the latitude range over which aircraft may be flown, often seen with wing mounted solar energy collection means, i.e. photovoltaic solar cells, characteristic of the prior art. During winter, at higher northern latitudes, the maximum angle of the sun above the horizon may be relatively small, and thus the effective collection area of the wing surface may be severely restricted.
While the Phillips reference alternatively suggests that solar cells may be placed on a tilting panel within a transparent fuselage structure, this arrangement would require the inclusion of a cooling system for the inner located cells, with the associated weight and aerodynamic drag penalties. The cooling requirement discussed in Phillips for maintaining high efficiency of inside-mounted cells is a generic limitation common to all photo-voltaic solar cell powered aircraft. This same limitation precludes the practical use of solar cells at the focus of a high concentration factor solar collector, since excessive heating of solar cells leads to substantially reduced efficiency.
Furthermore, the efficiency of photovoltaic electric energy collection, storage, and utilization in the prior art is relatively limited. Photovoltaic arrays of high efficiency are very expensive and tend to lose efficiency at elevated temperatures, and thus are not practical to use at the focus of a high flux solar concentrator. The prior art system of photovoltaic electric energy collection, storage, and utilization has a relatively small power to mass ratio. Thus the aircraft must typically fly at an altitude high enough to be above the clouds, and to avoid winds with velocities much higher than the airspeed of the vehicle, as described in the Phillips reference. Because of its long endurance and limited weight-carrying ability, this type of vehicle is normally considered to be a pilotless aircraft.
Various ground based solar energy collectors and concentrators, and interfaces to heat storage media and heat engines are also known. A few examples include: U.S. Pat. No. 4,586,334 to Nilsson, and U.S. Pat. No. 6,487,859 to Mehos. The Nilsson patent discloses “ . . . a solar energy power generation system which includes means for collecting and concentrating solar energy; heat storage means; Stirling engine means for producing power”, and “ . . . the means for collecting and concentrating solar energy is a reflective dish; and the heat transfer means includes first and second heat pipes; the heat storage means is preferably a phase change medium . . . ” The Mehos patent discloses: “ . . . sodium heat pipe receivers for dish/Stirling systems”, and cites references demonstrating: “ . . . sodium vapor temperatures up to 790° C.” Additionally, U.S. Pat. No. 4,125,122 discloses a heat pipe receiving energy from a solar concentrator, U.S. Pat. No. 6,700,054B2 describes connecting to a Stirling engine, among other things, and U.S. Pat. No. 4,088,120 describes a parabolic trough with a heat pipe at the focus connected to a heat storage medium. None of these representative references, however, disclose how the solar energy generation and storage system can be made sufficiently lightweight that it would be able to provide for the overnight propulsion of a solar-powered aircraft.
In addition, the utility of LiH as a thermal energy storage medium, i.e. a “thermal battery,” is known, and is based on the very high thermal energy per unit mass characteristic of LiH. For example, the specific energy released in the cooling of one kg of LiH from 1200 K to 600 K is 1900 W-hr. In contrast, lithium ion electrical storage batteries contain less than 10% as much energy per kg. Even a Hydrogen-Oxygen recyclable fuel cell with associated electrolyzer and gas storage contains no more than approximately 1000 W-hr per kg. It is appreciated that no other known solid, liquid, (or gaseous, if the mass of the requisite container is accounted for) compound has as high a specific thermal energy content as LiH for this temperature range. One example of LiH used as a thermal energy storage medium is disclosed in U.S. Pat. No. 3,182,653 to Mavleos et al. and directed to a Lithium hydride body heating device that uses LiH as a phase change medium to store heat energy for use in providing warmth to a diver. The '653 patent, however, does not disclose how highly reactive LiH may be safely contained for long periods of time. Theoretically, pure LiH has an infinite hydrogen vapor pressure just beyond the melting point of LiH. Thus, a container of LiH constructed according to the Mavleos disclosure, for example, may explode upon reaching the melting point of LiH at about 700° C.
In addition to the problems associated with photovoltaic power generation and solar energy collection/storage for solar aircraft described above, the operational requirements on the power plant of a solar thermal aircraft to enable it to remain aloft through the day/night cycle are extraordinarily demanding. Even lighter-than-air craft, by virtue of high altitude winds, must provide substantial propulsive power merely to maintain station above a point of interest. The power to mass ratio is therefore critical for solar thermal aircraft applications, since below a certain level, the aircraft cannot function as desired. As such, high efficiency heat engines are necessary for such solar thermal flight.
Various types of heat engines and power plants are known having various levels of efficiency. For example, one known type are Stirling engines, which when used in the context of parabolic dish solar concentrators are known to have achieved a thermal efficiency of over 40% for the conversion of heat to mechanical power.
Another type of heat engine, exemplified by the steam engine, is based on the Rankine thermodynamic cycle. Large ground-based steam turbine power plants have demonstrated even greater efficiencies than Stirling engines. For example, state of the art 1,050 MW Ultra Super Critical Steam Turbines available from GE, use steam at a temperature of 600° C. and pressure of 250 bar and have a demonstrated thermal efficiency of 49%. Although this higher efficiency would be very helpful to the performance of a solar powered aircraft, the GE turbines that produce it are enormous in size, due to the fact that the volume expansion ratio of ultra supercritical steam must be at least several thousand in order to attain high efficiency. In contrast to such large steam turbine systems, smaller gas turbine power plants generating below 100 kW typically achieve a thermal efficiency of only about 20%, according to the report “Efficiency in Electricity Generation.” And ground based power plants having a limited maximum temperature thermal reservoir, such as geothermal plants, have turned to the use of Organic Rankine Cycle alternatives to the conventional steam Rankine cycle. It is known that the Organic Rankine Cycle can enable reasonable thermal efficiency even at quite modest heating temperature. Such plants are typically limited in thermal efficiency to less than 20%.
And another type of heat engine is based on the Brayton thermodynamic cycle. For example, U.S. Pat. No. 3,708,979 entitled “Circuital Flow Hot Gas Engines,” incorporated by reference herein, discloses an improved form of closed cycle hot gas engine that was originally designed to operate approximately on a Brayton thermodynamic cycle, and that provides separate cylinders for the expander and compressor. A schematic illustration of the engine arrangement of the '979 patent is shown in FIG. 21 having valves in the gas flow circuit which define four separate volumes (with all valves closed) of working fluid and which control the flow of working fluid through the four volumes.
U.S. Pat. No. 3,376,706 discloses a power plant based on a Rankine-Brayton hybrid cycle, which compresses a working fluid above its critical pressure, then heated above its critical temperature, then expanded as a working gas in a turbine such that after expansion it is still above its critical temperature. The gas is then used in a heat exchanger to heat further gas, and then refrigerated to liquid state and recycled.
Accordingly, it is an object of the present invention to provide an aircraft powered by the heat of the sun.
Another object of the present invention is to provide a lightweight and highly efficient solar power plant and system for powering an aircraft by the heat of the sun.
Another object of the present invention is to provide an internally mounted solar power plant and system for powering an aircraft which does not require internal cooling.
And another object of the present invention is to provide a means for efficiently powering a solar aircraft by using a high efficiency heat engine, such as a Stirling engine.
Another object of the present invention is to provide a means for storing sufficient solar energy accumulated during the day to enable flight through the nighttime without excessive mass burden.
Another object of the present invention is to provide a means for maximizing solar energy collection and concentration by optimally aligning a heat collection element to the sun without re-orienting or otherwise changing the flight characteristics of the aircraft, e.g. banking.
Another object of the present invention is to provide a means for conserving heat energy during night time operation by preventing backflow of a heat transfer working fluid of a heat pipe.
And another object of the present invention is to provide a means for efficiently powering a solar aircraft by using a high efficiency heat engine, such as a Rankine-Brayton hybrid cycle engine.
These objects are achieved by the present invention described hereinafter.